The present invention relates to a mobile radio station for use in a business PHS (personal handyphone system).
Hereinafter, a conventional business PHS will be described. In the business PHS system, a plurality of connection or base stations (hereunder referred to as xe2x80x9cBSsxe2x80x9d) are connected to a PBX (private branch exchange). Each of the BSs is operative to intermittently transmit control data, which is represented by a control signal, in blocks to a mobile radio personal station (hereunder referred to as xe2x80x9ca PSxe2x80x9d) in a constant period of not less than 125 ms.
The control data comprises blocks respectively corresponding to several kinds of logical channels, such as BCCH (broadcast control channel), SCCH (signaling control channel) and PCH (paging channel). The block corresponding to the BCCH is periodically transmitted as the leading one of the blocks to be transmitted in a predetermined order. The logical channels respectively corresponding to these blocks represented by the control signal are referred to as xe2x80x9cLCCHsxe2x80x9d (logical control channels).
The PHS makes a search on the LCCHs, which are transmitted from the BSs, for a LCCH, which can be received by the PS. This search is referred to as a xe2x80x9cLCCH searchxe2x80x9d. Then, the PS performs the intermittent reception of a LCCH, which is selected as a result of the xe2x80x9cLCCH searchxe2x80x9d, with the timing of selection of this LCCH. This is referred to as xe2x80x9cLCCH (logical control channel) synchronization establishmentxe2x80x9d.
The frequency of the control signal is common to the BSs. Thus, each of the BSs carrier-senses at the initiation of transmission of the control signal so that no interference occurs between control signals respectively transmitted from the BSs. Consequently, no conflict occurs between the transmission times of control signals from the BSs.
The business PHS can achieve the parallel transmission of control signals, which represent the same control data and respectively have two different frequencies. In this case, the transmission times of the control signals of two frequencies are shifted from each other in the BS.
When the control signals having two different frequencies are transmitted from each of the BSs, the PS can achieve the LCCH synchronization establishment correspondingly to a given control signal from an arbitrary one of the BSs by performing the LCCH search.
Hereinafter, the LCCH search and the LCCH synchronization establishment to be performed in the PS will be described.
The PS receives control signals from a plurality of BSs. Therefore, immediately after the power is turned on, or when the PS moves into a service area from outside the service area (that is, from an area in which no radio waves transmitted from the BSs reach the PS), the PS performs the LCCH search so as to determine one of the BSs, which transmits a control signal that the PS should await. According to the PS-ID (identification information) peculiar to the PS, the PHS determines which of the two (control) frequencies of the control signals transmitted from each of the BSs is the (preferential) frequency of the control signals preferentially searched by the LCCH search. The preferential frequency and the other frequency are determined as first and second control frequencies, respectively. The PS decides to first perform the LCCH search on the control signals of the first control frequency.
The PS receives the control signals of the first control frequencies from the BSs and performs the LCCH search thereon. The PS chooses the BS having an ID, with which the PS can communicate, according to LCCH information. Then, the PS selects the LCCH having the highest electric field strength as an object LCCH from the corresponding LCCHs each having the electric field strength that is not lower than a predetermined threshold value. The LCCH synchronization establishment is performed on the selected LCCH.
When no object LCCH is found by the LCCH search performed correspondingly to the first control frequency, the PS performs the LCCH search on the control signals of the second control frequency.
If an object LCCH is found by the LCCH search performed correspondingly to the second control frequency, the LCCH synchronization establishment is performed correspondingly to the second control frequency.
Conversely, when no object LCCH is found by the LCCH search corresponding to the second control frequency, the PS performs an outside-the-service-area process to be performed when the PS is out of the service area.
In the case that the PS undergoes wave interference during the LCCH synchronization state thereof and thus an abnormality in the synchronization state occurs, the PS performs the following operation to be described hereinbelow.
In the case that the PS is in the LCCH synchronization establishment state at the first or second control frequency, when wave interference occurs, an abnormality in the LCCH synchronization state is caused.
The PS newly performs a LCCH search at the control frequency at which the LCCH synchronization establishment is achieved. When an object LCCH is found, the PS performs a LCCH synchronization establishment.
When no object LCCH is found, the PS performs a LCCH search at the other control frequency. Conversely, when an object LCCH is found, the PS performs the LCCH synchronization establishment at this control frequency.
Moreover, when no object LCCH is found, the PS performs the outside-the-area process.
Further, the related art is described in the Japanese Unexamined Patent Publication Nos. 9-322243, 4-371028 and 8-331628 Official Gazettes and xe2x80x9cSecond Generation Cordless Telephone System Standard Version 2 (Revision 2) RCR STD-28xe2x80x9d, Association of Radio Industries and Businesses, p. 59, June 1996.
However, the aforementioned conventional mobile radio station has the following drawbacks.
In the case that an abnormality in the LCCH synchronization state occurs owing to wave interference in the conventional mobile radio station, an operation similar to the LCCH search operation is performed so as to achieve LCCH resynchronization. Thus, it takes time for the PS to return to the LCCH synchronization establishment normal state.
When an abnormality in the LCCH synchronization state occurs, the PS is in an out-of-the-service-area state. In the case that BSs of different business PHSs are close to each other, the transmission times of signals between the BSs are asynchronous to each other. Consequently, the frequency of occurrences of wave interference between the control signals increases.
To prevent the interference between the control signals, for example, the Japanese Unexamined Patent Publication No. 9-322243 proposed a method for monitoring the interference between the control signals at the time of transmission thereof in the BSs and changing the transmission time of the control signal when an interference wave is detected.
However, although the aforementioned method achieves the avoidance of occurrences of the interference between the control signals, an abnormality in the LCCH synchronization state occurs in the PS having attained the LCCH synchronization establishment correspondingly to the control signal. Thus, the PS should newly perform a LCCH search. Consequently, it takes time to achieve the LCCH synchronization establishment again.
Accordingly, an object of the present invention is to provide a PHS mobile radio station, which can efficiently attain resynchronization when the wave interference between radio control signals sent from base stations (BS) occurs and no synchronization therebetween cannot be established.
To achieve the foregoing object, according to the present invention, there is provided a PHS mobile radio station, which has a memory unit for latching the time difference between the transmission times of control signals through logical control channels respectively corresponding to two control frequencies. During this radio station receives control signals in synchronization with the logical control channel (LCCH) corresponding to one of the two control frequencies, if an abnormality in the LCCH synchronization state occurs owing to wave interference, this mobile radio station controls a timing generation unit and a frequency changing unit on the basis of information memorized in the memory unit so that the reception of control signals is synchronized with the logical control channel corresponding to the other control frequency.